Field of the Invention
Embodiments of the present invention relate generally to testing of multi-chip integrated circuit packages and, more specifically, to a multi-chip package with selection logic and debug ports for testing inter-chip communication.
Description of the Related Art
In the packaging of integrated circuit (IC) chips, various packaging schemes are employed, including traditional two-dimensional (2D) integrated circuit (IC) packages as well as the more recently introduced 2.5D IC and 3D IC packages. In 2D IC packages, multiple chips are mounted on a printed circuit board, where high-performance logic, lower-performance logic, memory, and analog/RF functions, and other functional elements are presented as discrete devices in separate chip packages. By contrast, in 2.5D ICs and 3D IC packages, multiple IC chips are mounted on a silicon interposer instead of a conventional package substrate. The silicon interposer, which is typically a silicon wafer, allows very small and high-density conductive traces to be formed between the multiple IC chips because the fabrication processes used to form the conductive traces are the same processes used to form the metal interconnects in the metalization layers of a silicon chip.
Compared to 2.5D IC packages and 3D IC packages, a circuit board with individually packaged chips, such as a 2D IC package, has numerous disadvantages. For example, a 2D IC package is generally larger, heavier, consumes more power, and, because the signals propagate relatively slowly across the circuit board from one chip to another, is slower than an equivalent 2.5D or 3D IC package. Furthermore, a 2D IC package has more possible points of failure, given that the soldered joints on the circuit board are more likely to fail than the electrical connections formed within an interposer. That said, troubleshooting a 2D IC package after the different chips have been mounted on the circuit board is relatively straightforward. In particular, the conductive traces carrying I/O signals between the various chips on the circuit board are easily accessible and therefore can be employed to measure specific I/O signals during troubleshooting.
By contrast, troubleshooting a 2.5D or 3D IC package is far more problematic because the I/O signals transmitted between the different chips typically are embedded in the silicon interposer and are not physically accessible. Furthermore, because 2.5D and 3D IC packages are high-bandwidth and are quite dense, typically implementations can include thousands of conductive traces routed between the different chips. One example of such an implementation is a memory bus residing in between a processor and a high-bandwidth memory chip. In such implementations, even if the traces could be physically accessed through the silicon interposer with a probe, the accurate and reliable selection of a specific conductive trace or combination of conductive traces for the purpose of troubleshooting the IC package would be very difficult, if not impossible.
As the foregoing illustrates, what would be useful is a technique for measuring inter-chip communications within a multi-chip package.